Light-emitting diode (LED) is widely used as a solid state light source. Light-emitting diode (LED) generally comprises a p-type semiconductor layer, an n-type semiconductor layer, and an active layer between the p-type semiconductor layer and the n-type semiconductor layer for emitting light. The material of the LED generally comprises group III-V compound semiconductor such as gallium phosphide, gallium arsenide, or gallium nitride. The principle of the LED is to transform electrical energy to optical energy by applying electrical current to the LED and injecting electrons and holes to the active layer. The combination of electrons and holes in the active layer emits light accordingly.